Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
Description of the Related Art
Recently HDR image data has come to be used widely due to the improvement in the performance of image sensors, advancement in technology related to image processing, and the like. HDR image data is image data corresponding to a high dynamic range (HDR). For example, HDR image data is image data of which color depth is greater than 8 bits. And the dynamic range is, for example, a range of possible gradation values or a range of possible brightness.
As the performance of the display apparatus improves, techniques to implement a high brightness display have started to be proposed. For example, along with the improvement in the performance of light emitting diode (LED), which is used for the light source of a backlight unit for a liquid crystal display apparatus, techniques to implement a high brightness display by controlling the emission brightness of the backlight unit are being proposed. A technique to implement a high brightness display for each of a plurality of pixels independently using an organic light emitting diode (OLED; an organic electro luminescence (EL) element) has also been proposed.
For example, when HDR image data is created, assuming that this image data is displayed on a display apparatus which can implement a high brightness display, a user who forms the image creates the HDR image data such that a small sized white image stands out on the screen. In concrete terms, the user determines the brightness change between frames, the brightness change between scenes, the maximum brightness of each frame, the maximum brightness of each scene and the like, so that a small sized white image stands out on the screen. In future, it is expected that HDR image data, to be displayed on a display apparatus which can implement a high brightness display, will be popular.
HDR image data is generated based on various standards, such as SMPTE ST2084 and Hybrid Log Gamma (HLG). HLG is a standard in which the brightness value is handled as a relative brightness defined by a gamma curve, which can support conventional TV apparatuses as well. SMPTE ST2084 is a brightness information recording standard for the content of HDR, which is standardized by SMPTE, and SMPTE ST2084 is a standard in which brightness up to 10,000 nit is handled as the absolute brightness. In SMPTE ST2084, the correspondence between a gradation value and brightness is defined using a perceptual quantizer (PQ) curve. A characteristic of the SMPTE ST2084 and HLG is that the brightness of the HDR image data is handled as the display brightness of the display apparatus.
HDR10 is a standard defined in SMPTE ST2084. In HDR10, maximum content light level (MaxCLL) and maximum frame average light level (MaxFALL) are attached as meta data, which is static data. MaxCLL is meta data which defines the maximum brightness of the HDR image data, and MaxFALL is meta data which defines the average brightness of the HDR image data. The authoring guide line of HDR10 specifies that the image region exceeding 1000 nit is limited to a small sized image region, such as an image region of specular reflection. It also specifies that the average brightness is not more than 400 nit.
Further, as the standard of the HDR image data, a new standard, other than the above mentioned standards, may be independently defined by a camera manufacturer, a display manufacturer, an image distributor or the like in the future, and the HDR image data may be generated using such an independent standard.
The HDR image data, which has a plurality of standards and a plurality of systems, is accessible to a user via a network, or is recorded on a Blu-ray disk or the like. When HDR image data is increasingly made available to users, HDR image data will likewise be displayed more frequently on display apparatuses.
In some cases, a plurality of images based on a plurality of image data is displayed on the screen of the same display apparatus. A technique to display a plurality of images on the same screen is disclosed in, for example, Japanese Patent Application Publication No. 2009-005258. According to the technique disclosed in Japanese Patent Application Publication No. 2009-005258, image processing that is performed on image data of each one of the plurality of images is changed in accordance with the display size (size of the image on the screen). The plurality of image data may include conventional standard dynamic range (SDR) image data, which is defined in the REC. 709 and of which the upper limit of brightness is 100 nit, and HDR image data. The plurality of image data may also include two or more HDR image data, of which standards and upper limits of brightness and the like are different from each other. In such cases, a plurality of image processing operations having mutually different correspondences of gradation value of the image data and the display brightness of the display apparatus (brightness on screen) may be performed for the plurality of image data respectively.